1. Field of the Invention
This invention relates to polyester compositions, and particularly to poly(cyclohexylenedimethylene terephthalate) (PCT) copolyester formulations having improved crystallization behavior and articles made therefrom exhibiting improved shatter resistance. More particularly, the invention relates to PCT copolyester formulations prepared by the process of solid state polymerization and have greater than 70 mole % 1,4-cyclohexanedimethanol, a crystallization halftime, as defined herein, of between 2 minutes and 10 minutes, and an inherent viscosity of greater than 0.90 dL/g, and to extrusion blow molded articles made from the PCT copolyester formulations. Surprisingly, these articles have improved shatter resistance over articles made from poly(ethyleneterephthalate) (PET) and copolyesters containing terephthalic acid, ethylene glycol and less than about 40 mole % 1,4-cyclohexanedimethanol (CHDM).
2. Background of the Invention
Extrusion blow molding is a common process for creating hollow articles from polymeric materials. A typical extrusion blow-molding manufacturing process involves: 1) melting the resin in an extruder; 2) extruding the molten resin through a die to form a parison having a uniform wall thickness; 3) clamping a mold having the desired finished shape around the parison; 4) blowing air into the parison, causing the extrudate to stretch and expand to fill the mold; 5) cooling the molded article; and 6) ejecting the article from the mold.
The hollow articles generated by extrusion blow molding are often used to contain solid or liquid products. The container must, therefore, be sufficiently tough to protect the product and prevent it from leaking or spilling after an accidental drop or impact. Toughness of the blow molded article is related to several factors, including part design, wall thickness, size of the container, and material. For filled articles, size of the container affects toughness greatly, as the weight of the contents produces the impact weight. Larger containers will hold heavier masses that will produce a higher impact load. In order to compensate for these higher impact loads, wall thickness must be increased or a tougher material must be selected. Unfortunately it is not always possible to increase wall thickness due to melt strength limitations and cost. Thus, the preferred solution is usually to extrusion blow mold the containers from a tougher material.
For many applications, such as non-carbonated beverage bottles and other containers, the polymer used in the extrusion blow molded articles also need to be amorphous so that the blow molded article will be transparent. This limits the number of polymers that can be utilized. Polyethylene terephthalate (PET) and copolymers based on PET are often used for extrusion blow-molding hollow, amorphous transparent articles.
Unfortunately, the low toughness of PET based polyesters and PET copolyesters containing up to 40% CHDM restricts container design and utility. It is possible to add impact modifiers to these materials, but the resultant blends are generally opaque. There are other clear extrusion blow moldable materials available, such as PVC and polycarbonate. However, these resins can have problems with chemical resistance, toughness, resin cost and environmental concerns. Thus, there is a need for a copolyester having greater toughness than PET and PET copolymers that can be extrusion blow molded into hollow, amorphous transparent articles.
It is known that injection molded articles made from copolyesters of terephthalic acid with ethylene glycol and greater than 50 mole % 1,4-cyclohexanedimethanol, such as those described in U.S. Pat. No. 2,901,466, show improved toughness over injection molded articles made from PET and PET copolymers. Typically, these copolyesters have inherent viscosities (I.V.) less than 0.90 dL/g, and require high melt processing temperatures and fast quenching to avoid crystallization related problems. This combination of low I.V. and high melt temperature processing leads to a low melt strength in the polyester or copolyester. Additionally, articles that crystallize during the extrusion blow molding process are either totally or partially white. Such white or hazy parts are unacceptable in applications where clarity and transparency are required.
In order to form good quality containers that have uniform side wall thickness and to prevent tearing (blow-out) of the parison during expansion, the polymer extrudate must have good molten dimensional stability. Dimensional stability is related to the polymer""s melt strength. Generally, melt strength has been determined in accordance with ASTM D3835 by extruding the molten polymer downward through a die 0.1 inch (0.254 cm) in diameter and 0.25 inch (0.635 cm) long at a shear rate of 20 secondsxe2x88x921 using an Instron rheometer and allowing the extrudate to fall freely. A material having high melt strength has a tendency to resist stretching and flowing as a result of gravitational force when in the softened or molten state. Shorter flow lengths indicate better melt strength and, consequently, less sag of the parison. Thus, materials with high melt strength perform better in the extrusion blow molding process.
The melt strength of a polymer is directly related to its melt viscosity measured at 1 radian/second on a rotary melt rheometer. Polymers that have melt strengths high enough to be extrusion blow molded typically have melt viscosities of greater than 30,000 poise measured at 1 rad/sec and at the melt temperature of a typical parison.
Because of the high viscosity requirements particular to extrusion blow molding, special grades of PET and PET copolymers must be used. The inherent viscosity of a PET based polyester does not exceed 0.90 dL/g when made in a typical commercial melt phase polymerization reactor. In order to obtain the high melt viscosities required for the extrusion blow molding process, a PET based polyester with an inherent viscosity of less than 0.90 dL/g must be processed at a relatively cold temperature. For example, a melt phase PET copolyester having terephthalic acid with ethylene glycol and containing between 20 and 50 mole % 1,4-cyclohexanedimethanol as a modifying diol needs to be extrusion blow molded at a parison melt temperature of between 210xc2x0 C. and 230xc2x0 C. Fortunately, this copolyester has a low melting temperature and very slow crystallization halftime.
In contrast, if PET composed substantially of terephthalic acid and ethylene glycol or PET with low levels, i.e., less than about 10 mole % and preferably less than about 5 mole % of a secondary comonomer such as a diacid, a diol or combinations thereof, is processed at this low a temperature, it can crystallize in the feed section or barrel of the extruder, or in the parison. Crystallization in the extruder can halt the process or lead to imperfections in the final article. Crystallization in the parison can lead to undesirable opacity or embrittlement of the final article, or will lead to a parison that can not be blown into the desired final shape. Thus, other methods must be used to raise the melt viscosity of crystallizable polyesters so that they can be processed at temperatures above their melt temperature. One method is to add a branching agent to the composition. Another method is to raise the molecular weight through solid state polymerization, referred to herein as solid state or stating polymerization processing.
The solid state polymerization process is well known. Typically, amorphous precursor pellets that have been prepared by melt phase polymerization are first crystallized at a temperature 10xc2x0-100xc2x0 C. below their melt temperature and then moved to a second stage where they are held at a temperature at least 10xc2x0 C. below their melt temperature for a sufficiently long time (2-40 hours) in a vacuum to increase the inherent viscosity of the polymer.
One unique problem with polyesters and copolyesters is that they must be both solid state polymerization processed and thereafter extrusion blow molded into amorphous articles is that the precursor polyester pellets must crystallize quickly enough so that they can be crystallized in the solid stating process, but not crystallize so rapidly as to crystallize during the extrusion blow molding process. Accordingly, there is a limited range of compositions that will meet this criteria.
U.S. Pat. No. 3,117,950 discloses the use of the solid stating polymerization process to increase the inherent viscosity of PCT based polymers from an I.V. of less than 0.57 to a greater I.V. This patent does not disclose the specific compositions disclosed herein nor the use of these solid stated polymers for extrusion blow molding to produce transparent extrusion blow molded products.
U.S. Pat. No. 4,983,711 discloses copolyesters of terephthalic acid with 25 to 75 mole % ethylene glycol and 75 to 25 mole % CHDM that contain a branching agent. These compositions are useful for extrusion blow molding. However, this patent does not disclose the use of solid state polymerization to obtain high molecular weight copolyesters suitable for extrusion blow molding applications.
Briefly, the present invention is a copolyester composition having at least about 70 mole % terephthalic acid and least about 70 mole % 1,4-cyclohexanedimethanol wherein the mole percentages of the acid component total 100 mole % and the mole percentages of the glycol component total 100 mole %; and having an inherent viscosity greater than 0.9 dL/g as determined in a 60/40 (wt./wt.) phenol/tetrachloroethane at a concentration of 0.5 g/100 ml as determined at 25xc2x0 C. The copolyester composition is prepared by solid state polymerizing a copolyester composition having a starting inherent viscosity of from about 0.4 to about 0.8 dL/g for about 1 minute to about 100 hours and at a temperature of from about 140xc2x0 C. to about 2xc2x0 C. below the melting point of the polyester so that the inherent viscosity is increased to greater than 0.9 dL/g. After solid state polymerization, the copolyester of the present invention has a crystallization halftime of between 2 minutes and 10 minutes, when measured by from the glass state by a DSC halftime technique at a temperature of 170xc2x0 C. The copolyester typically has a melt temperature of from about 240xc2x0 to about 270xc2x0 C. when measured by a DSC scan rate technique.
Another aspect of the present invention is for a blow molded article, such as food containers, personal care containers, medical devices and containers, industrial containers, as well as blow molded parts for appliances, cabinetry and automotive applications made from the solid state polymerized copolyester. The copolyester produces a amorphous extrusion blow molded article that is tougher, i.e., better impact resistance, than an article made from PET or PET based copolyesters having less than 70 mole % 1,4-cyclohexanedimethanol (PETG).
The extrusion blow molded articles made from the compositions described in greater detail herein are amorphous and are tougher than articles made from PET or PETG. The copolyester of the invention typically contains residue moieties of a diacid and a diol wherein the mole percentages expressed herein are based on 100 mole percent of the diacid and 100 mole percent of the diol. In accordance with the present invention, the copolyester contains at least about 70 mole % terephthalic acid as the acid moiety and at least about 70 mole % 1,4-cyclohexanedimethanol as the diol moiety and is solid state polymerized to an inherent viscosity of greater than about 0.90 dL/g, and preferably to an inherent viscosity of from about 0.95 to about 1.10 dL/g after solid stating. The copolyester has a melt temperature of from about 240xc2x0 to about 270xc2x0 C. when measured by a DSC heating scan rate technique. The inherent viscosities of the copolyesters of this invention are determined in a 60/40 (wt./wt.) phenol/tetrachloroethane at a concentration of 0.5 g/100 ml as determined at 25xc2x0 C. The acid component of the copolyesters comprises at least about 70 mole % terephthalic acid. Desirably, the acid moiety may contain repeat units of from 0 to about 30 mole % of a secondary acid selected from the group of dibasic acids of terephthalic acid, isophthalic acid, cyclohexanedicarboxylic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, stilbenedicarboxylic acid and mixtures thereof. When using the cyclohexanedicarboxylic acids, they may be in the cis or trans forms or as cis/trans isomer mixtures. When cyclohexanedicarboxylic acid is used, 1,3- and 1,4-cyclohexanedicarboxylic acid are preferred. When naphthalenedicarboxylic acid is used, 2,6-, 2,7-, 1,4- and 1,5-naphthalenedicarboxylic acid are preferred. Other acids include aliphatic dicarboxylic acids and cycloaliphatic dicarboxylic acids, each preferably having 4 to 40 carbon atoms, such as phthalic acid, cyclohexanediacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid and mixtures thereof. The mole percentages of the acid component of the copolyesters of the invention equal a total of 100 mole %.
The copolyesters also comprise at least 70 mole % of 1,4-cyclohexanedimethanol. The cyclohexanedimethanol may be in the cis or trans forms or as cis/trans isomer mixtures. Desirably, the diol moiety may contain repeat units of from 0 to about 30 mole % of a secondary diol selected from aliphatic or alicyclic glycols, preferably containing 2 to 20 carbon atoms. These remaining or secondary diols may be selected from ethylene glycol, diethylene glycol, triethylene glycol, propanediol, butanediol, pentanediol, hexanediol, tetramethylcyclobutanediol, 1,4-cyclohexanedimethanol and mixtures thereof. The mole percentages of the glycol component of the copolyesters of the invention equal a total of 100 mole %.
In a preferred embodiment, the copolyesters have at least about 70 mole % terephthalic acid and from about 20 to about 30 mole % isophthalic acid and 100 mole % 1,4-cyclohexanedimethanol in a 70/30 cis/trans ratio.
In a second preferred embodiment, the copolyesters have 100 mole % terephthalic acid and from about 77 mole % to about 90 mole % 1,4-cyclohexanedimethanol in a 70/30 cis/trans ratio and from about 23 to about 10 mole % ethylene glycol and more preferably from about 15 mole % to about 20 mole % ethylene glycol and the remainder is 1,4-cyclohexanedimethanol.
The copolyesters of this invention are readily prepared using melt phase or solid state polycondensation procedures described in greater detail in U.S. Pat. Nos. 4,256,861, 4,539,390, and 2,901,466, the entire disclosures of which are incorporated herein by reference. The copolyester may be made by batch or continuous processes and include preparation by direct condensation or by ester interchange.
Briefly, a typical procedure consists of at least two distinct stages; the first stage, known as ester-interchange or esterification, is conducted under an inert atmosphere at a temperature of 150xc2x0 C. to 250xc2x0 C. for 0.5 to 8 hours, preferably from 180xc2x0 C. to 240xc2x0 C. for 1 to 4 hours. The glycols, depending on their reactivities and the specific experimental conditions employed, are commonly used in molar excesses of 1.05-2.5 per total moles of acid-functional monomers. The second stage, referred to as polycondensation, is conducted under reduced pressure at a temperature of 230xc2x0 C. to 350xc2x0 C., preferably 265xc2x0 C. to 325xc2x0 C., and more preferably 270xc2x0 C. to 300xc2x0 C. for 0.1 to 6 hours, and preferably 0.25 to 2 hours. Stirring or appropriate reaction conditions are used in both stages to ensure adequate heat transfer and surface renewal of the reaction mixture. The reactions of both stages are facilitated by appropriate catalysts, especially those well-known in the art, such as alkoxy titanium compounds, alkali metal hydroxides and alcoholates, salts of organic carboxylic acids, alkyl tin compounds, metal oxides, and combinations thereof.
The copolyester precursor particles are normally crystallized under forced motion at a temperature of about 100xc2x0 C.-260xc2x0 C. prior to being solid-state polymerized. In some processes, the crystallization and solid-state polymerization steps might not be distinct.
Solid-state polymerization is a process well known in the art. For example, U.S. Pat. No. 4,064,112, which is incorporated herein by reference, describes a typical solid-state process. In this process, amorphous precursor pellets that have been prepared by melt phase polymerization are first crystallized at a temperature 10xc2x0-100xc2x0 C. below their melt temperature (crystallization phase) and then further held at a temperature of at least 10xc2x0 C. below their melt temperature for a sufficiently long time, e.g., 2-40 hours, in the presence of either vacuum or dry nitrogen to increase their IV (solid stating phase). These high temperatures are required to allow polymerization to proceed at a relatively rapid and economical rate. At these high temperatures, amorphous pellets would soften and fuse together into a highly viscous block. In contrast, crystalline pellets will not stick together at these temperatures. Thus, solid state polymerization can only be performed on crystallized pellets. Copolyesters possessing crystallization halftimes, measured at 170xc2x0 C., greater than 10 minutes will not fully crystallize in the crystallization phase and then will undesirably fuse together in the solid stating phase.
Generally when molding grade pellets are produced, either a batch or continuous process is used. In a batch process, pellets are added to a large container heated according to the two stage process described above. The container is continuously rotated to provide uniform heating of the pellets, and to prevent sticking of the pellets to the container walls during the initial crystallization. In a continuous process, the pellets first drop by gravity into a crystallizer unit, and then flow by gravity through a large heated container which builds the IV. Continuous processes are preferred for commercial operations for reasons of economics. Normally, in solid stating pellets in accordance with the present invention, particles of regular or irregular shape may be used. The particles may be of various shapes and sizes such as spherical, cubical, irregular, cylindrical, and shapes which are generally flat.
Solid stating in accordance with the present invention is accomplished by subjecting the copolyester particles to a temperature of about 140xc2x0 C. to about 2xc2x0 C., preferably about 100xc2x0 C. to about 10xc2x0 C., below the melting point of the copolyester. The time of solid stating can vary over a wide range (about 1 minute to 100 hours) according to temperature to obtain the desired I.V., but with the higher temperatures, usually about 10 hours to about 60 hours is sufficient to obtain the desired I.V. During this period of solid stating, it is conventional to flow a stream of inert gas through the pellets to aid in temperature control of the copolyester pellets and to carry away reaction gases such as ethylene glycol and acetaldehyde. Preferably, the inert gas is recycled for economic reasons. Inert gases which may be used include helium, argon, hydrogen, nitrogen and mixtures thereof. It should be understood that the inert gas may contain some air.
It is often observed during solid stating polymerization processing that the rate of inherent viscosity increase may slow with time. Thus, the maximum IV that can be obtained may be limited by the initial IV of the precursor material. For this reason, the precursor I.V. for the copolyesters described in this invention should be between 0.4 and 0.9 dL/g, preferably between 0.6 and 0.85 dL/g, most preferably between 0.65 and 0.8 dL/g.
These compositions can be extrusion blow molded using conventional equipment known to those skilled in the art such as continuous, reciprocating screw and accumulator head extrusion blow molding equipment. Copolyesters that possess crystallization halftimes, measured at 170xc2x0 C., of less than 2 minute will fully or partially crystallize during cooling in the mold, resulting in undesirable whiteness in the final part.
In accordance with another aspect of the present invention, the copolyester of the present invention can be blow molded into various articles and containers, such as for example, food containers, personal care containers, medical devices and containers, industrial containers, as well as blow molded parts for appliances, cabinetry and automotive applications. Accordingly, the blow molded article is composed of at least 50 weight %, more preferably at least 75 weight % and most preferably at least about 95 weight % of the copolyester composition described in detail above.
The following terms and ranges useful in understanding the examples of the invention are defined below.
Processability was determined using a 80 mm Bekum H-121 continuous extrusion blow molding machine fitted with either a barrier screw or a mixing screw containing both cavity transfer and Maddock mixing sections. The extruder was run at 6 rpm. The materials were extruded into 340 milliliter (ml) Boston Round Bottles. The bottles weighed between 25 and 30 grams. As defined herein, the xe2x80x9cminimum processing temperaturexe2x80x9d is the lowest temperature at which a parison could be extruded without observation of gels or crystallization in the side walls or at the pinch point. It is possible that even at this minimum processing temperature the parison would lack sufficient melt strength to successfully blow a bottle. xe2x80x9cSuccessfully blowing a bottlexe2x80x9d is defined as blowing a bottle into the desired shape with uniform side wall thickness and without the formation of holes.
Toughness was measured by filling a successfully blown Boston Round bottles with water, storing them overnight, capping them and then dropping them from a series of heights of from about 2 feet to about 11 feet in accordance with ASTM D2463. The 50% drop height was determined in accordance with ASTM D2463 using procedure B, the Bruceton Staircase method. Boston Round bottles (340 ml) made from the copolyesters of the present invention have drop heights greater than 9 feet.
Transparency as defined by the present invention is measured according to ASTM Method D1003. It is preferable that molded objects prepared from the copolyester of the invention have a diffuse transmittance value of less than about 60%, more preferably, less than about 40%, and more preferably, less than about 20%. When the diffuse transmittance value is less than about 60%, the molded objects are visually clear.
Crystallization half times from the glass state, as defined by the present invention, are measured using a Perkin-Elmer Model DSC-2 differential scanning calorimeter. A sample of 15.0 mg was sealed in an aluminum pan and heated to 290xc2x0 C. at a rate of about 320xc2x0 C./minute and held for 2 minutes to uniformly melt the material. The sample is then cooled to below its glass transition temperature at a rate of about 320xc2x0 C./minute to generate an amorphous specimen. The specimen was then reheated at 320xc2x0 C./minute immediately to the predetermined isothermal crystallization temperature in the presence of helium. The crystallization half time was determined as the time span from reaching the isothermal crystallization temperature to the point of a crystallization peak on the DSC curve. Crystallization halftimes from the glass transition temperature in this invention are measured on the precursor pellets. This provides an indication of the ability of the precursor pellets to be crystallized in commercial solid stating equipment. Half times of less than 10 minutes indicate that the material can be crystallized in a solid stating process. These halftimes also correlate with halftimes measured on the final extrusion blow molding parison. The parison half times provide an indication of whether the parison will crystallize during processing. Half times of greater than 2 minutes indicate that the parison can be processed in an extrusion blow molding process to produce amorphous bottles. For forming satisfactory blow molded articles, the copolyesters of the present invention must have crystallization half times from the glass state of about 2 minutes to about 10 minutes.
Melt viscosities were measured in accordance with ASTM D4440. A frequency scan of between 1 rad/sec and 400 rad/sec was employed. The melt viscosity at 1 rad/sec is correlated to the xe2x80x9cmelt strengthxe2x80x9d of the polymer. Preferred copolyesters of the invention must have a melt viscosity at the xe2x80x9cminimum processing temperaturexe2x80x9d of at least 30,000 poise.
Inherent viscosities (I.V., dL/g) as defined by the present invention are measured at 25xc2x0 C. using 0.5 g polymer per 100 mL of a solvent consisting of 60 parts by weight phenol and 40 parts by weight tetrachloroethane. Copolyesters of the present invention should have inherent viscosity (I.V.) values of about 0.9 to about 1.3 dL/g.
Melt temperatures were determined using differential scanning calorimetry in accordance with ASTM D3418. A sample of 15.0 mg was sealed in an aluminum pan and heated to 290xc2x0 C. at a rate of 20xc2x0 C./minute. The sample was then cooled to below its glass transition temperature at a rate of about 320xc2x0 C./minute to generate an amorphous specimen. The melt temperature, Tm, corresponds to the peak of the endotherm observed during the scan. Note that some copolyesters do not exhibit a melt temperature as defined by this method. The melt temperature of a copolyester helps define the xe2x80x9cminimum processing temperaturexe2x80x9d of the copolyester. Preferred copolyesters should have melt temperatures (Tm) of between about 240xc2x0 C. to 275xc2x0 C. as determined by Differential Scanning Calorimetry (DSC) (ASTM D3418) at a scan rate of 20xc2x0 C./min.
The present invention is illustrated in greater detail by the specific examples presented below. It is to be understood that these examples are illustrative embodiments and are not intended to be limiting of the invention, but rather are to be construed broadly within the scope and content of the appended claims.